Radio frequency transmitter and methods thereof

ABSTRACT

In one embodiment, the present invention provides a radio frequency transmitter that may have a processor and a controller that reduce current consumption of the power amplifier of the radio frequency transmitter.

CROSS-REFERENCE

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/769,444, filed on Jan. 26, 2001, now U.S. Pat. No.6,587,511, issued on Jul. 1, 2003.

BACKGROUND OF THE INVENTION

[0002] Modern systems enable radio transmitters to transmit at reducedpower for long periods of time. The modulating signal of thesetransmissions may have large peak-to-minimum amplitude variations. Sincethe efficiency of power amplifiers is generally reduced atless-than-maximum power levels, these two factors may increase theaverage current consumption of power amplifiers in radio transmitters.

[0003] There is a continuing need to reduce the current consumption ofpower amplifiers in radio transmitters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The subject matter regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of operation, together with objects, features and advantagesthereof, may best be understood by reference to the following detaileddescription when read with the accompanying drawings in which:

[0005]FIG. 1 is a schematic block-diagram illustration of an exemplaryradio frequency transmitter, according to an embodiment of the presentinvention;

[0006]FIGS. 2A and 2B are schematic illustrations of signal spacediagrams, helpful in understanding the present invention;

[0007]FIG. 3 is a schematic block-diagram illustration of an exemplaryup-conversion chain, according to an embodiment of the presentinvention;

[0008]FIGS. 4A, 4B and 4C are exemplary graphical illustrations of theinstantaneous efficiency of the radio frequency transmitter of FIG. 1and of a conventional class-B power amplifier as a function of theinstantaneous output signal power due to the amplitude of the modulatingsignal;

[0009]FIG. 5 is a schematic block-diagram illustration of an exemplaryradio frequency transmitter, according to another embodiment of thepresent invention;

[0010]FIG. 6A is an exemplary graphical illustration of theinstantaneous efficiency of the radio frequency transmitter of FIG. 1for a constant envelope signal as a function of the output signal power,according to another embodiment of the present invention; and

[0011]FIG. 6B is an exemplary graphical illustration of theinstantaneous efficiency of the radio frequency transmitter of FIG. 1for a non-constant envelope signal as a function of the output signalpower, according to a further embodiment of the present invention.

[0012] It will be appreciated that for simplicity and clarity ofillustration, elements shown in the figures have not necessarily beendrawn to scale. For example, the dimensions of some of the elements maybe exaggerated relative to other elements for clarity. Further, whereconsidered appropriate, reference numerals may be repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0013] In the following detailed description, numerous specific detailsare set forth in order to provide a thorough understanding of theinvention. However it will be understood by those of ordinary skill inthe art that the present invention may be practiced without thesespecific details. In other instances, well-known methods, procedures,components and circuits have not been described in detail so as not toobscure the present invention.

[0014] It should be understood that the present invention may be used ina variety of applications. Although the present invention is not limitedin this respect, the circuit disclosed herein may be used in manyapparatuses such as in the transmitters of a radio system. Radio systemsintended to be included within the scope of the present inventioninclude, by way of example only, cellular radiotelephone communicationsystems, two-way radio communication systems, one-way pagers, two-waypagers, personal communication systems (PCS), and the like.

[0015] Types of cellular radiotelephone communication systems intendedto be within the scope of the present invention include, although arenot limited to, Direct Sequence—Code Division Multiple Access (DS-CDMA)cellular radiotelephone communication systems, Wideband CDMA (WBCDMA)and CDMA2000 cellular radiotelephone systems, Global System for MobileCommunications (GSM) cellular radiotelephone systems, North AmericanDigital Cellular (NADC) cellular radiotelephone systems, Time DivisionMultiple Access (TDMA) systems, Enhanced Data for GSM Evolution (EDGE)and Universal Mobile Telecommuniuications Systems (UMTS).

[0016] Reference is now made to FIG. 1, in which an exemplary radiofrequency (RF) transmitter in accordance with an embodiment of thepresent invention is described. An RF transmitter 100 may comprise adigital signal processor (DSP) 102, baseband (BB) to RF up-conversionchains 104 and 106, RF preamplifiers 108 and 110, a power amplifier 112,an antenna 114 and a controller 116.

[0017] A BB input signal 118 may be provided to DSP 102, which mayconvert it into two constant envelope vectors according to a methodwhich will be described hereinbelow with respect to FIGS. 2A and 2B. Forexample, the first constant envelope vector may be represented bybaseband signals I₁ and Q₁, while the second constant envelope vectormay be represented by baseband signals I₂ and Q₂. Up-conversion chain104 may convert signals I₁ and Q₁ into an RF signal RF₁; similarlyup-conversion chain 106 may convert signals I₂ and Q₂ into an RF signalRF₂. RF signals RF₁ and RF₂ have a common carrier frequency. Anexemplary embodiment of up-conversion chains 104 and 106 is describedhereinbelow with respect to FIG. 3, although the present invention is inno way limited to this particular exemplary embodiment.

[0018] RF preamplifier 108, which has a variable gain, may amplifysignal RF₁ to produce a signal RF_(IN-1); similarly RF preamplifier 110,which has a variable gain, may amplify signal RF₂ to produce a signalRF_(IN-2). Power amplifier 112, which may have reactive termination, mayamplify and combine RF_(IN-1) and RF_(IN-2) to produce an output signalRF_(OUT) for transmission by antenna 114.

[0019] Power amplifier 112 may comprise two branch amplifiers 120 and122 connected in parallel, and shunt reactance elements 124 and 126 atthe output of branch amplifiers 120 and 122, respectively. B_(S) denotesthe shunt reactance of element 124 and −B_(S) denotes the shuntreactance of element 126. The efficiency of power amplifier 112 at aspecific output signal power may be improved by adjusting the shuntreactance B_(S). Power amplifier 112 may also comprise atransmission-line-coupler 128 for combining the outputs of branchamplifiers 120 and 122. Transmission-line-coupler 128 may comprise twotransmission lines 130 and 132 connected to antenna 114 so that the sumof the branch currents goes through the load. Other combiner schemesyielding the same performance may be implemented instead, namely hybridBALUN, center tap inductor, etc.

[0020] Controller 116 may receive as input a targeted average outputsignal power level P. Targeted average output signal power level P maybe selected from a range of power levels or may be selected from adiscrete set of at least two power levels. Controller 116 may providedata related to P to any of DSP 102, up-conversion chains 104 and 106,and RF preamplifiers 108 and 110, with the result that power amplifier112 may produce an output signal whose average power is substantiallyequivalent to P. The operation of controller 116 and DSP 102 is betterunderstood if reference is made additionally to FIGS. 2A and 2B, whichare schematic illustrations of signal space diagrams. The horizontalaxis of the diagram represents the real (in-phase) component of a signalvector, while the vertical axis of the diagram represents the imaginary(quadrature) component.

[0021] Three concentric circles, 202, 204 and 206, are shown in FIG. 2A.A vector 208 from the center of the diagram to the largest circle 206represents the amplitude and phase of a BB signal, which afterup-conversion and amplification may produce a signal having a maximalinstantaneous output signal power. This maximal instantaneous outputsignal power may be determined both by the maximum amplitude A_(MAX) ofinput signal 118 (FIG. 1) and by the maximum average power P_(MAX) thatpower amplifier 112 may be able to produce. Similarly, a vector 210 fromthe center of the diagram to circle 202 represents the amplitude andphase of a BB signal, which after up-conversion and amplification mayproduce a signal at an instantaneous output signal power that may bedetermined both by the minimum amplitude A_(MIN) of input signal 118 andby an average output signal power level P_(TH), the determination ofwhich will be explained hereinbelow.

[0022] According to some embodiments of the present invention, when thetargeted average output signal power level P is between P_(MAX) andP_(TH), controller 116 may provide DSP 102 with ρ(P) so that DSP 102 mayrepresent a baseband vector 212 by two constant envelope vectors 214 and216. Since baseband vector 212 has an average amplitude controlled byρ(P), baseband vector 212 may result, after up-conversion andamplification, in a signal at an average output signal power P and at aninstantaneous output signal power determined both by the instantaneousamplitude A (t) of input signal 118 and by the targeted average outputsignal power level P.

[0023] The radius of circle 204 is predetermined both by the maximumamplitude A_(MAX) and by ρ(P_(MAX)). The data flow from controller 116to DSP 102 is indicated in FIG. 1 by line 133, and constant envelopevectors 214 and 216 are represented by the signals I₁ and Q₁ and I₂ andQ₂, respectively.

[0024] If BB input signal 118 at time t is denoted s(t), with the real(in-phase) component denoted I(t) and the imaginary (quadratire)component denoted Q(t), then the following decomposition holds:

s(t)=I(t)+jQ(t)

[0025] The instantaneous amplitude A (t) of input signal 118 at time tis given as follows: ${A(t)} = {\sqrt{{I^{2}(t)} + {Q^{2}(t)}}.}$

[0026] Signals I₁ and Q₁ are then given by Equations 1A and 1B, asfollows: $\begin{matrix}{{{I_{1}(t)} = {\sqrt{\rho (P)}\left( {{I(t)} - {{Q(t)}\sqrt{\frac{{\rho \left( P_{MAX} \right)} \cdot A_{MAX}^{2}}{{\rho (P)} \cdot {A^{2}(t)}} - 1}}} \right)}},} & \text{(Eq. 1A)} \\{{{Q_{1}(t)} = {\sqrt{\rho (P)}\left( {{Q(t)} - {{I(t)}\sqrt{\frac{{\rho \left( P_{MAX} \right)} \cdot A_{MAX}^{2}}{{\rho (P)} \cdot {A^{2}(t)}} - 1}}} \right)}},} & \text{(Eq. 1B)}\end{matrix}$

[0027] and signals I₂ and Q₂ are given by Equations 2A and 2B, asfollows: $\begin{matrix}{{{I_{2}(t)} = {\sqrt{\rho (P)}\left( {{I(t)} + {{Q(t)}\sqrt{\frac{{\rho \left( P_{MAX} \right)} \cdot A_{MAX}^{2}}{{\rho (P)} \cdot {A^{2}(t)}} - 1}}} \right)}},} & \text{(Eq. 2A)} \\{{Q_{2}(t)} = {\sqrt{\rho (P)}{\left( {{Q(t)} - {{I(t)}\sqrt{\frac{{\rho \left( P_{MAX} \right)} \cdot A_{MAX}^{2}}{{\rho (P)} \cdot {A^{2}(t)}} - 1}}} \right).}}} & \text{(Eq. 2B)}\end{matrix}$

[0028] It will be appreciated by persons of ordinary skill in the artfrom Equations 1A, 1B, 2A and 2B that the amplitude of the signalrepresented by I₁ and Q₁, namely $\sqrt{I_{1}^{2} + Q_{1}^{2}},$

[0029] and the amplitude of the signal represented by I₂ and Q₂, namely$\sqrt{I_{2}^{2} + Q_{2}^{2}},$

[0030] are both equal to$\sqrt{\rho \left( P_{MAX} \right)} \cdot {A_{MAX}.}$

[0031] It will also be appreciated by persons of ordinary skill in theart that the relative phase differences of these signals are determinedfrom the instantaneous amplitude of input signal 118 and from thetargeted average output signal power level P. Clearly the presentinvention is not limited in any way to the exemplary equations givenhereinabove in Equations 1A, 1B, 2A and 2B. Rather, any other set ofequations yielding a constant envelope signal represented by signals I₁and Q₁, and a constant envelope signal represented by signals I₂ and Q₂,is clearly also within the scope of the present invention.

[0032] According to some embodiments of the present invention, when thetargeted average output signal power level P is between P_(MAX) andP_(TH), controller 116 may provide predetermined, fixed values to anyamplification elements of up-conversion chains 104 and 106 and to RFpreamplifiers 108 and 110. The data flow from controller 116 toup-conversion chains 104 and 106 are indicated in FIG. 1 by lines 134and 136, respectively. Lines 138 and 140 indicate the data flow fromcontroller 116 to RF preamplifiers 108 and 110, respectively.

[0033] Three concentric circles, 202, 204 and 218, are shown in FIG. 2B.Circles 202 and 204 are the same or similar to those shown in FIG. 2A. Avector 220 from the center of the diagram to circle 218 represents theamplitude and phase of a BB signal, which after up-conversion andamplification may produce a signal at an instantaneous output signalpower that may be determined both by the maximum amplitude A_(MAX), ofinput signal 118 (FIG. 1) and by the average output signal power levelP_(TH).

[0034] According to some embodiments of the present invention, when thetargeted average output signal power level P is less than P_(TH),controller 116 may provide DSP 102 with the power ρ(P_(TH)) so that DSP102 may represent a baseband vector 222 by two constant envelope vectors224 and 226, where the size of constant envelope 204 is the same orsimilar to that used in FIG. 2A. Constant envelope vectors 224 and 226may be represented by signals I₁ and Q₁, and I₂ and Q₂, respectively,where Equations 1A, 1B, 2A and 2B are used with ρ(P_(TH) in place ofρ(P). However, baseband vector 222, after up-conversion andamplification at fixed gain values, would produce an output signal at anaverage output signal power, which may be determined both by theinstantaneous amplitude A(t) of input signal 118 and by thepredetermined power level P_(TH), and which is higher than the targetedaverage output signal power level P. Therefore, controller 116 mayreduce the amplitudes of signals I₁ and Q₁, and I₂ and Q₂, or may reducethe gain of any of variable amplification elements in up-conversionchains 104 and 106 and RF preamplifiers 108 and 110, or a combinationthereof, with the result that power amplifier 112 may produce an outputsignal whose average power is substantially equivalent to P.

[0035] The predetermined average output signal power level P_(TH) mayact as a threshold between two modes of operation of the RF transmitter,according to some embodiments of the present invention. In one mode, theRF transmitter may control the instantaneous output signal power bycombining constant envelope signals whose relative phase differences aredetermined from the instantaneous amplitude of a baseband input signaland from the targeted average output signal power level P, and byup-converting at a fixed gain. In another mode, the RF transmitter maycontrol the instantaneous output signal power by combining constantenvelope signals whose relative phase differences are determined fromthe instantaneous amplitude of the baseband input signal and from thepredetermined average output signal power level P_(TH), and byup-converting at a variable gain which is dependent on the targetedaverage output signal power level P and which is lower than the fixedgain of the first mode. Alternatively, in this other mode, the RFtransmitter may control the instantaneous output signal power bycombining constant envelope signals whose relative phase differences aredetermined from the instantaneous amplitude of the baseband input signaland from the predetermined average output signal power level P_(TH), andwhose amplitudes have been reduced in the baseband according to thetargeted average output signal power level P, so that the average powerof the output signal is substantially equivalent to the targeted averageoutput signal power level P.

[0036] Reference is now made to FIG. 3, which is a schematicblock-diagram illustration of an exemplary up-conversion chain,according to an embodiment of the present invention. The up-conversionchain may comprise an intermediate frequency (IF) local oscillator (LO)300 and an RF local oscillator 302, IQ modulators 304 and 306, and phaselock loops (PLL) 308 and 310.

[0037] IQ modulator 304 may comprise mixers 312 and 314 and combiner316. Mixer 312 may receive as input I₁ and sin(ω_(1F)t), where ω_(1F)denotes the frequency generated by IF LO 300 and t denotes time. Mixer314 may receive as input Q₁ and cos(ω_(1F)t). Combiner 316 may combinethe outputs of mixers 312 and 314, and provides the combination to PLL308. Similarly, IQ modulator 306 may comprise mixers 318 and 320 andcombiner 322. Mixer 318 may receive as input I₂ and sin(ω_(1F)t). Mixer320 may receive as input Q₂ and cos(ω_(1F)t). Combiner 322 may combinethe outputs of mixers 318 and 320, and provides the combination to PLL310.

[0038] PLL 308 may comprise a phase detector (PD) 324, a loop filter 326and a voltage-controlled oscillator (VCO) 328. PLL 308 may also comprisea mixer 330, mixing the output of VCO 328 with the signal produced by RFLO 302, and providing an IF modulated signal to PD 324. Similarly, PLL310 may comprise a PD 334, a loop filter 336 and a VCO 338. PLL 310 mayalso comprise a mixer 340, mixing the output of VCO 338 with the signalproduced by RF LO 302, and providing an IF modulated signal to PD 334.

[0039] Alternatively, the up-conversion chain may comprise variableamplifiers (not shown) that amplify the input signals I₁ and Q₁, and I₂and Q₂, prior to their modulation by IQ modulators 304 and 306,respectively. The gain of these variable amplifiers may be reduced bycontroller 116 (not shown) when the targeted average output signal powerlevel P is less than the predetermined power level P_(TH).

[0040] Reference is now made to FIGS. 4A, 4B and 4C, which are exemplarygraphical illustrations of the instantaneous efficiency of the radiofrequency transmitter of FIG. 1 (indicated by a solid line) and of aconventional class-B power amplifier (indicated by a dotted line) as afunction of the output signal power. In FIG. 4A the average outputsignal power (indicated by a circle) is P_(MAX), and the instantaneousoutput signal power (indicated by the solid and dotted lines) variesaccording to the amplitude of the input signal. In FIG. 4B the averageoutput signal power is P_(TH), and in FIG. 4C the average output signalpower is less than P_(TH). The average current consumption of the RFtransmitter of FIG. 1 may be appreciably improved with respect to thatof class-B power amplifiers.

[0041] As shown in the exemplary graphical illustrations of FIGS. 4B and4C, P_(TH) is chosen to be the average output signal power at which theefficiency has a peak value. However, it will be appreciated that thereare many other ways to select the threshold P_(TH), all of which areincluded in the scope of the present invention. For example, thethreshold P_(TH) may be chosen by minimizing the current consumptionaccording to the output signal power probability distribution and theamplitude distribution of the baseband input signal.

[0042] Reference is now made to FIG. 5, which is a schematicblock-diagram illustration of an exemplary radio frequency transmitter,according to another embodiment of the present invention.

[0043] An RF transmitter 500 may comprise DSP 102, RF preamplifiers 108and 110, power amplifier 112, antenna 114 and controller 116. As in FIG.1, BB input signal 118 may be provided to DSP 102. RF transmitter 500may also comprise IF local oscillator 300, RF local oscillator 302, IQmodulators 304 and 306, and PLLs 308 and 310.

[0044] RF transmitter 500 may also comprise a feedback path tocompensate for circuit imperfections that may occur in an open looparrangement such as that of FIG. 1. In this embodiment, DSP 102 maycomprise a compensation module 502. A small portion of the transmittedsignal RF_(OUT) may be taken through a directional coupler 504 via astep attenuator 506. The state of step attenuator 506 may be controlledby controller 116, as indicated by line 507, in order to divide theentire dynamic range into several smaller regions. The output of stepattenuator 506 passes through an image rejection mixer (IRM) 508. IRM508 down-converts the RF signal to IF. IRM 508 may receive as input, inaddition to the RF signal, a signal from RF local oscillator 302. The IFsignal produced by IRM 508 may be demodulated by an I/Q demodulator 510,which may receive as input a signal from IF local oscillator 300. I/Qdemodulator 510 may produce feedback signals I_(FB) and Q_(FB), whichmay be provided to DSP 102 through analog-to-digital converters (notshown).

[0045] As indicated by line 133, controller 116 may provide DSP 102 witha power level ρ. As explained hereinabove, when the targeted averageoutput signal power level P is in a first range of average output signalpower levels, i.e. between P_(MAX) and P_(TH), then the power level ρ isrelated to the targeted average output signal power level P. When thetargeted average output signal power level P is in a second range ofaverage output signal power levels, i.e. less than P_(TH), then thepower level ρ is related to the predetermined average output signalpower level P_(TH).

[0046] Compensation module 502 may compare the input signal 118, thefeedback signals I_(FB) and Q_(FB), the power level ρ and the state 507of step attenuator 506 to create the compensated baseband signals I₁ andQ₁, and I₂ and Q₂.

[0047] RF transmitter 500 may also comprise a power level measurementunit 512 that may take a small portion of the output of step attenuator506 through a directional coupler 514. Power level measurement unit 512may provide a measured power level P_(FB) to controller 116. Controller116 may compare the targeted output signal power level with measuredpower level P_(FB) in order to set the targeted amplification values forRF preamplifiers 108 and 110 and for the amplification elements in theup-conversion chains.

[0048] In another embodiment of the present invention, signals I₁ and Q₁are given by Equations 3A and 3B, as follows: $\begin{matrix}{{{I_{1}(t)} = {\sqrt{\rho (P)}\left( {{I(t)} - {{Q(t)}\sqrt{\frac{\rho \left( P_{MAX} \right)}{\rho (P)} - 1}}} \right)}},} & \text{(Eq. 3A)} \\{{{Q_{1}(t)} = {\sqrt{\rho (P)}\left( {{Q(t)} + {{I(t)}\sqrt{\frac{\rho \left( P_{MAX} \right)}{\rho (P)} - 1}}} \right)}},} & \text{(Eq. 3B)}\end{matrix}$

[0049] and signals I₂ and Q₂ are given by Equations 4A and 4B, asfollows: $\begin{matrix}{{{I_{2}(t)} = {\sqrt{\rho (P)}\left( {{I(t)} + {{Q(t)}\sqrt{\frac{\rho \left( P_{MAX} \right)}{\rho (P)} - 1}}} \right)}},} & \text{(Eq. 4A)} \\{{Q_{2}(t)} = {\sqrt{\rho (P)}{\left( {{Q(t)} - {{I(t)}\sqrt{\frac{\rho \left( P_{MAX} \right)}{\rho (P)} - 1}}} \right).}}} & \text{(Eq. 4B)}\end{matrix}$

[0050] It will be appreciated by persons of ordinary skill in the artfrom Equations 3A, 3B, 4A and 4B that the amplitude of the signalrepresented by I₁ and Q₁, namely $\sqrt{I_{1}^{2} + Q_{1}^{2}},$

[0051] and the amplitude of the signal represented by I₂ and Q₂, namely$\sqrt{I_{2}^{2} + Q_{2}^{2}},$

[0052] are both equal to $\begin{matrix}{\sqrt{{\rho \left( P_{MAX} \right)} \cdot \left( {I^{2} + Q^{2}} \right)}.} & \quad\end{matrix}$

[0053] That is, their amplitude depends on the amplitude of the inputsignal and the maximal average output signal power and does not dependon the average output signal power. These signals are constant envelopesignals only if the input signal is a constant envelope signal. It willalso be appreciated by persons of ordinary skill in the art that therelative phase differences of these signals are determined from thetargeted average output signal power and not from the instantaneousamplitude of input signal 118.

[0054] According to this embodiment, if the amplitude of baseband inputsignal 118 is constant, an exemplary graphical illustration of theefficiency of the radio frequency transmitter of FIG. 1 as a function ofthe output signal power is shown in FIG. 6A, to which reference is nowmade. The efficiency has a peak at two output signal powers. Thepredetermined average output signal power level P_(TH) may be set to beclose to the lower of these output signal powers having a peakefficiency.

[0055] If the amplitude of baseband input signal 118 is not constant,the amplitudes of baseband signals I₁ and Q₁, and I₂ and Q₂, aredetermined from the instantaneous amplitude A(t) of baseband inputsignal 118, and the relative phase differences of baseband signals I₁and Q₁, and I₂ and Q₂, are determined from the targeted output signalpower. The average efficiency of the radio frequency transmitter of FIG.1 for this embodiment as a function of the output signal power is shownin FIG. 6B.

[0056] While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

What is claimed is:
 1. A method comprising: when a targeted power levelis below a predetermined power level: generating baseband signals havingrelative phase differences, said relative phase differences determinedfrom an instantaneous amplitude of an input signal and from saidpredetermined power level, the amplitude of said baseband signals beingdetermined, at least in part, from said targeted power level; andcombining signals derived from said generated signals into an outputsignal having an average power that is substantially equivalent to saidtargeted power level, said derived signals having a common carrierfrequency.